Before inoculation into products, such as food products or dietary supplements, bacteria are cultured in order to provide a suspension containing large amounts of bacteria. The suspension is usually concentrated using centrifugation, filtration, distillation, sedimentation or flocculation. This concentration step is often followed by freezing or freeze-drying or drying or storage of the microbial concentrate as a frozen product in liquid nitrogen to preserve and/or store the bacteria.
However, freeze-drying of the bacteria is a bottleneck in the industrial production of storable viable bacteria due to the cell damage and loss of viable cells during the freeze-drying but also due to the long process time, usually days, thus resulting in high cost industrial freeze drying processes. This is primary due to the primary drying, or ice sublimation, stage of freeze drying process which is frequently the most time consuming portion of the process (PIKAL M. J.; SHAH S.). This is due to the practice to keep the products at a low product temperature during primary freeze-drying in order to avoid product collapse i.e. loss in particle microstructure which is regarded as determinable for products quality and stability (Schersch et al. 2256-78). Product collapse is usually avoided commercially by applying mild process conditions i.e. low pressure and low shelf/heating plate temperature during freeze-drying, which results in prolonged drying cycles.
Therefore, there is still a need to improve the efficiency of freeze drying methods suitable for bacteria-containing suspensions, to obtain a highly concentrated bacteria suspension with a limited loss of biological activity as well as a limited loss of viable bacteria both during the production and the subsequent storage. These methods need to be feasible at any scale, but especially on the industrial scale, where large volumes of suspension are concentrated.